It is necessary to provide a non-sticking interface between a nano-structured master and the polymer to be embossed in the replication of nano-structures by imprinting processes, such as hot embossing. This allows demolding without degrading the replication fidelity. Adhesion during the mold release may cause damage to the replica. Further, residual polymer structures may contaminate the surface of the mold. A prerequisite for a successful imprint process is that the mold used should be chemically and mechanically stable and should also adhere poorly to polymers such that sticking to the mold is avoided.
During the last years a lot of effort has been made to minimize the adhesion forces at the stamp/polymer interface in nano-imprint lithography [1-3] mainly with the establishment of thin fluorinated films deposited onto the stamp surfaces [4-7]. These anti-sticking films lead to the reduction of surface energy of the stamps and improve the release of the stamp from the imprinted polymer. It is well known that for gold surfaces, the adsorption of fluorinated alkyl thiol self-assembled monolayer (SAM) films leads to both highly hydrophobic and lipophobic surfaces demonstrating contact angles with water of 120° and with hexadecane of 81° [8]. U.S. Pat. Nos. 5,512,131 and 6,380,151 disclose the use of SAM forming species as part of a method of patterning a material surface for use in microelectronic and optoelectronic applications. Neither of these patents however, utilise the SAM films for the purpose of reduction of adhesive forces.
When addressing the reduction of adhesive forces, two alternative solutions presently show promise. The first alternative involves the deposition of polytetrafluoroethylene (PTFE) films onto structured stamps with the help of a CF4/H2 or CBF3 plasma [4]. In this case, the polymer is only bound weakly to the substrate and the film has to be renewed after a few imprints in order to maintain the imprint quality. The second alternative involves the formation of anti-sticking films of fluorinated alkyl silanes using chemical vapour deposition possessing film thicknesses in the monomolecular region [5-7]. The direct chemical bond of the silane groups, especially for silicon stamps, with the substrate surfaces results in good imprint results of patterns with feature sizes down to 20 nm [5].
From the literature it is also known that alkyl phosphate salts, alkyl phosphoric acids, and alkyl phosphonic acids produce well-ordered self-assembling monolayers on oxidized surfaces of aluminium, tantalum, niobium, zirconium, and titanium taking advantage the strong ionic linkage between the phosphate group and the oxidized metal surface [9-13]. After heating of the samples, the films demonstrate a strong resistance against mechanical force as well as chemical stability against solvents [14]. Also fluorinated alkyl phosphoric acid derivates have been recently deposited onto oxidized aluminium and alumina surfaces [15,16].
With regards to substrates, the application of polycrystalline nickel sheets instead of silicon wafers as stamp materials may be regarded as an industrially more relevant alternative. Nickel (Ni) stamps are commonly used in several industrial applications of nano-imprint lithography (NIL).
Ni especially may be regarded as a highly suitable material for the preparation of phosphoric acid based surfactants in an industrially relevant process. The ionic nature of Ni-oxide increases the possibility of an ionic linkage between the phosphate group and the Ni stamp surface without the application of additional adhesion promoters, allowing imprints of patterns with feature sizes down to 100 nm. The ionic nature of the Ni-oxide surface is thus considered suitable for the application of acid-based surfactants or surface modification agents while discouraging the use of silane agents, which in their turn, are more applicable when silicon stamps or silicon substrate surfaces are used [5, 17-19].
WO 2004/000567 A1 discloses anti-sticking films deposited on Ni stamps, used in nanoimprint lithography. Phosphate groups are among others used as linkage groups to metal films (Ti, Zr, Nb, Ta, Al and a mixture thereof) deposited as adhesion promoters onto Ni stamps (claims 3, 2, 10).